This invention relates to telecommunications line interface circuits.
In line interface circuits for two-wire and four-wire telecommunications lines, e.g. telephone lines, it is common to provide a transformer in view of its desirable common mode signal rejection and ground isolation characteristics. In telephone applications such lines usually must be able to conduct a substantial direct current, typically up to about 60 mA, which also flows through a primary winding of the transformer. In addition, a line terminating impedance, typically of 600 to 900 ohms, is reflected from the secondary to the primary winding of the transformer to match the impedance of the line.
To achieve a desired low cut-off frequency of 50 Hz or less, the primary winding of such a transformer must provide an inductance of several Henries, necessitating a large number of turns of the primary winding even using a ferrite core transformer. To avoid magnetic flux saturation of the transformer core as a result of the direct current flowing through this large number of turns, the transformer must be physically large, and consequently expensive. The transformer size also creates a significant problem in trying to provide compact arrangements of many line interface circuits.
U.S. Pat. No. 4,982,426 discloses a circuit arrangement for a line interface circuit in which the transformer may be reduced in size without adversely affecting the performance of the circuit. However, in common with prior art circuit arrangements, the performance of the circuit at low frequencies is limited by the transformer parasitics, namely its magnetizing inductance.